Impact of the Pacific Meridional Mode on Hawaiian Rainfall Variability

Abstract

The Pacific meridional mode (PMM) features air–sea interactions within the subtropical and tropical eastern North Pacific; however, its influence on Hawaiian rainfall variability remains elusive. This study aims to elucidate PMM-related large-scale atmospheric patterns and their repercussions on island rainfall dynamics across diverse time scales. In reference to the peak PMM season in boreal spring, we delineate distinct atmospheric patterns during the antecedent winter and concurrent spring. Our analysis employs multiple linear regression and disentangles the roles of El Niño–Southern Oscillation (ENSO) and the PMM in driving rainfall variability. ENSO emerges as the primary driver of winter rainfall variability, while the PMM assumes a pivotal role in spring rainfall, particularly impacting the southern islands. During a (1) PMM phase in winter, anomalous surface westerly winds decelerate prevailing trade winds, engendering an east–west anomalous rainfall dipole pattern across the Hawaiian Islands. This, in turn, amplifies median and lower-quartile seasonal rainfall over the typically arid leeward sides of the island chain. Subsequently, in spring, a (1) PMM intensifies moisture and ascent over the tropical eastern North Pacific, precipitating extensive rainfall across the state. For disturbance-driven rainfall, a (1) PMM coincides with heightened frontal-related rainfall, whereas El Niño is associated with decreased rainfall from Kona lows, upper-level lows, and trade-wind events. Our evaluation of PMM-related daily rainfall intensity underscores spatial variations: A (2) PMM corresponds to reduced moderate daily rainfall over windward sides, potentially exacerbating drought occurrences. Conversely, leeward sides experience an increase in extreme rainfall events in both winter and spring during a (1) PMM, suggesting a heightened risk of floods.